A number of Japanese clocks have apertures in their front panel to indicate the date. They are driven by a single peg in the day going dial or hand on the front of the clock. These indicators may be single or dual.
Clocks with a single aperture normally show the twelve terrestrial branches of the Zodiac and repeat five times in the 60 day month.
Double Date Apertures
Terrestrial + Celestial Stems: Some other Japanese clocks have double date apertures where the second aperture shows the ten celestial stems of the Zodiac and repeat five times in the 60 day month.
Tidal Indicator: The Hashira dokei in the possession of the author has an aperture providing a 15 day indication of the high tide cycle (two per day). In Shinto religious practices, some of which still survive, are worship to the Spirits of the Sea. Practices related to the Spirits of the Sea, were the requirement that certain ceremonies be performed either, when the tide was coming in, or at flood tide – and may originate from the positive action of the rise in tides – rather than the ebb or outflow. The ceremonies in which the state of the tide was considered were (i) naming a child on its birthdate, (ii) giving a child symbolic gifts at its first birthday, (iii) the marriage ceremony of san san-kudo, and (iv) the moving of a younger son, on his marriage, from the main house of his father to his own house. No doubt there were many other occasions, such as the instance in which the boar is allowed with the sow in heat only when the tide was coming in.
It was necessary, therefore, to be able to compute the state of the tide quickly and easily. These were generally given from an almanac, where a figure was obtained which gives the portion of an hour to be added to 12 o’clock on the first day of the fifteen-day lunar cycle, two of which make up a lunar month. The value varies with geographical position, but typically for Minatogawa it was 0.8 hours, or forty-eight minutes, for low tide; everyone seemed to have this figure at his fingertips. On the first day of the fifteen-day cycle, low tide is forty-eight minutes after 12 o’clock; on the eighth day, it would be 6:24 a.m. (8 days times 0.8 hours after 12). High tide is computed by adding or subtracting six hours.